Steel cords formed from twisted steel wires are used as reinforcing material for rubber products such as tires, conveyor belts, and rubber hoses. One example is a stranded steel cord in which a plurality of sheath strands is twisted around the outer circumferential surface of a core strand (see, for example, Japanese Unexamined Patent Application Publication No. 2012-36539A).
A steel cord 12 having a 7×7 structure like that illustrated in FIG. 4 is often used as a core in a conveyor belt. The 7×7 steel cord 12 is formed by six sheath strands 14 twisted around the outer circumference of a single core strand 13. The core strand 13 is formed by six steel wires 13b twisted around a single steel wire 13a. The sheath strands 14 are formed by six steel wires 14b twisted around a single steel wire 14a. Thus, three twining processes are required to manufacture the 7×7 steel cord 12, leading to the problem of very poor productivity if the cord is small in diameter. If the steel cord is used as a core in a conveyor belt, superior cord strength is required, as is reduced weight and improved bending resistance in order to reduce the energy consumption needed to drive the belt.